1. Field of the Invention
The present invention relates to a planar light source device, a liquid crystal display device, and a television set, and more particularly, to a technology of using a light guide plate to obtain planar light emission from a non-planar light source such as a light emitting diode (LED).
2. Description of the Related Art
Liquid crystal display devices are used for personal computer displays, liquid crystal television sets, and the like. In the liquid crystal display device, each pixel of a liquid crystal panel for forming an image does not emit light by itself. Accordingly, the liquid crystal display device includes a light source for irradiating the liquid crystal panel with light, and the image formed by the liquid crystal panel is made visible for display by means of transmitted light or reflected light from the liquid crystal panel. An example of the light source that can be used is a planar light source called a backlight device disposed behind the liquid crystal panel. In this case, an image is displayed by the transmitted light from the liquid crystal panel.
The backlight device typically uses a large number of point light sources or linear light sources (hereinafter, collectively referred to as non-planar light sources). The backlight device spreads light emitted from the non-planar light sources in a planar manner by means of a light guide plate or the like, and extracts the light from one surface (exit-side surface) of the light guide plate. The planar light source device using the light guide plate is divided into a so-called edge-light type or side-light type in which the non-planar light sources are disposed on a side end surface of the light guide plate, and a so-called direct type in which the non-planar light sources are arrayed along an opposite surface of the exit-side surface (incident-side surface) of the light guide plate. As the non-planar light sources, cold cathode fluorescent lamps (CCFLs) or LEDs are used.
FIG. 13 is a vertical cross-sectional view schematically illustrating a structure of a conventional direct type planar light source device. In the direct type planar light source device, a plurality of light emitting elements 4 are disposed on a substrate 6, and a light guide plate 2 is stacked on the substrate 6. In the structure illustrated in FIG. 13, a recess portion 8 is formed in a lower surface of the light guide plate 2, and the light emitting elements 4 arrayed on the substrate 6 are each accommodated within the recess portion 8. On the other hand, a recess portion 10 having a tapered shape is formed in an upper surface of the light guide plate 2 at a position immediately above the light emitting element 4. Inclined surfaces constituting the tapered shape are formed so as to satisfy conditions of total reflection for light which is emitted from the light emitting element 4 and directs upward. That is, the light is totally reflected on the inclined surfaces so as not to transmit through the light guide plate 2 but to be guided into the light guide plate 2. FIG. 14 is a partial vertical cross-sectional view schematically illustrating another structure of the conventional direct type planar light source device. Also in this structure, a tapered recess portion 10 is formed in the upper surface of the light guide plate 2 at the position immediately above the light emitting element 4 so that the light which is emitted from the light emitting element 4 and directs upward may be reflected into the light guide plate 2, to thereby prevent the transmission of the light. The light that has been guided into the light guide plate 2 is repeatedly totally reflected on the upper and lower surfaces so as to spread within the plane. Further, a white reflective film for diffusing and reflecting light is formed on the lower surface of the light guide plate 2 in the form of halftone dots so that a light component that exits from the upper surface of the light guide plate 2 may be generated at the portion in which the white reflective film is formed. The density and size of the halftone dots at the respective points on the lower surface are designed so that the intensity of light emission from the upper surface of the light guide plate 2 may be uniform.